The present invention relates to a detector for ionizing particles, especially muons.
Mu mesons or muons are created in the upper atmosphere by cosmic rays. Muons reaching the earth's surface have very high energies, typically 3 to 4 Gev, and are capable of penetrating substantial thickness of objects. Muons have much greater penetrating power than x-rays, so the detection of muons can be used to determine the internal structures of large or dense objects. Muons have been used to reveal the internal structures of pyramids [see L. W. Alvarez, et. al., Search for Hidden Chambers in the Pyramids Using Cosmic Rays, Science 167 (1970) 832.] and volcanoes [see H. Tanaka, et al., Development of a two-fold segmented detection system for near horizontally cosmic-ray muons to probe the internal structure of a volcano, Nuclear Instruments & Methods in Physics Research A, 507 (2003) 657].
Recently, there has been interest in muon detection techniques for inspecting cargo for nuclear materials. L. J. Schultz [see Cosmic Ray Muon Radiography, PhD thesis, Portland State University (2003), http://www.cgsd.com/ref/Schultz.pdf] explains the principles involved in producing an image from muon detection data, known as muon radiography. There are three basic approaches. These are (1) to measure the momentum lost by muons passing through objects, (2) to measure the relative numbers that fail to emerge, called “range out.” from the objects, and (3) to measure the relative scattering of muons passing through an object.
All three methods require muon detection, and in particular the determination of the trajectory of muons potentially traversing the objects being imaged. Blanpied, et al, [see Radiographic Imaging with Cosmic Ray Muons, slide presentation, Los Alamos National Laboratory, March 2005, http://www.cgsd.com/ref/Blanpied.pdf] states that measuring the momentum loss is too expensive with present technology and opines that the scattering technique is best for cargo inspection while the range out method is best for studying geological formations.
Gustafsson [see Tomography of canisters for spent nuclear fuel using cosmic ray muons, Uppsala University Neutron Physics Report UU-NF 05#08, October 2005, http://www.cgsd.com/ref/Gustafsson.pdf] provides an analysis of a proposed use of the scattering method for inspection purposes.
For the scattering and range out methods the practice is to use two parallel planar arrays of detectors separated by a few feet. An arriving muon is detected in each plane nearly simultaneously, and the x,y coordinates in each plane used to determine the three-dimensional line of travel. The accuracy of the determination is limited by the resolution of the detector arrays.
Geiger counters are known in the prior art as devices used to detect muons. In principle, an array of Geiger tubes in a plane could be used to determine the coordinates of an arriving muon that passes through one of the tubes in the array. Further, Geiger-Muller tubes are a type of gas detector in which ions created by the energetic muon are detected by the current flow between anode and cathode in the tube created by the ionization. The ionization can also be detected optically by photographing the ionization trail in a drift chamber, or by using a photomultiplier tube to detect the light emitted during ionization. While such devices are known and foreseeable methods to detect muon positions, the high cost to assemble a commercially viable unit is a limitation of these detection methods.
An alternative to gas detectors is to use crystals that emit light when struck by a muon. Each crystal is paired with a photomultiplier that detects the light path. The preferred crystalline material is CZT. M. L. McConnel et al., [see “Three-dimensional imaging and detection efficiency performance of orthogonal coplanar CZT strip detectors,” Proc. SPIE, Vol. 4141 (2000) 157]. Both the crystal material and the photomultipliers are costly. An advantage of crystal-based detectors is that the sensitivity of the sensor cells is quite uniform because the size and properties of the crystals are uniform.
Yet another approach is to use a series of longitudinal detectors side by side to determine the muon coordinates. In the version by Schulz, long Geiger-Muller tubes are laid side by side in a plane. Each tube has a central anode wire. A muon strike causes a current pulse which travels to the ends of the tube. A timing measurement determines the point along the tube of the ionization. Thus, which tube is struck determines one coordinate of the arrival in the plane of the detectors, and the timing measure determines the other coordinate.
Mockett et al, in U.S. Pat. No. 4,504,438, also uses a combination of longitudinal detectors and timing, but in a cylindrical arrangement for use in determining geological formations around a bore hole. The invention determines one coordinate, the azimuth, by determining which of the parallel vertical wires is conducting current. The other coordinate, the vertical displacement, is determined by timing the propagation of the pulse detected concurrently in a helically wound wire. Concentric detectors are used to determine the three-dimensional trajectory of the muon.
A position sensing photomultiplier tube incorporating a wire grid anode is known in prior art. [see Knoll, Glenn F.; Radiation detection and measurement, 3rd Edition, Wiley, New York, 2000 pp 300-302]. This detector uses a photocathode sheet. Both the horizontal and vertical wires in the anode grid are charged with the anode voltage. However, it detects photons in the ultraviolet spectrum and cannot detect muons.
More recently, Shpantzer et al in US20070102648A1 [Method and system for nuclear substance revealing using muon detection] discloses a concealed nuclear material detection system which compares actual muon coordinate and actual incidence angle with predicted coordinate and incidence angle for each muon for detecting nuclear materials. The US20070102648A1 application uses a prior art muon detector in a system for relatively small objects that is unrelated to the muon detector invention disclosed herein. US20070102648A1 depends upon the deflection angle and not upon whether or not the muon was absorbed by the cargo under scrutiny. Further, the position of the object within the space must be well defined to calculate the deflection angle. The present invention is distinguished because it makes no assumptions as to object size and can be used to reconstruct a three-dimensional representation of the objects in the cargo.
Finally, Morris et al in US20080191133A1 [RADIATION PORTAL MONITOR SYSTEM AND METHOD] discloses a portal monitoring system for inspecting occupied vehicles which has cells with operating gas, and detecting sources where the system detects materials or devices occupying the inspection volume from multiple scattering of particles. The US20080191133A1 patent application is on a detection system using drift cell muon detectors. Drift cell muon detectors are known in prior art. The drift cell detector is essentially Geiger tubes having a central conductor and an outer conducting tube shell. The present invention is distinguished in that it is a flat plate detector and does not use either tubes or an outer conducting surface. Further, the present invention discloses a detector using a wire grid and non-conducting outer plates whereas the US20080191133A1 application does not use plates, a grid, or non-conducting outer surfaces and is inherently more expensive to construct and less accurate in muon detection.
Muon detectors are known in the art and prior art detectors function to detect muons and to determine their trajectories. The limitations are in the size, resolution, and cost of the detectors in the prior art.
Therefore, it is an object of the present invention to provide a muon detector having high resolution.
It is a further object of the invention to provide a detector that can be constructed at low cost.
It is a further object of the invention to provide a volumetrically scalable detector system that is a plurality of low cost detectors arranged in correspondent surface modules.
It is a further object of the invention to provide a detector that provides uniform sensitivity from detector cell to detector cell.